kbuild: link-vmlinux.sh: simplify .version increment

Since commit 1f2bfbd00e46 ("kbuild: link of vmlinux moved to a
script"), it is easy to increment .version without using a temporary
file .old_version.

I do not see anybody who creates the .tmp_version.  Probably it is a
left-over of commit 4e25d8bb9550fb ("[PATCH] kbuild: adjust .version
updating").  Just remove it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/Makefile b/Makefile
index cf007a3..9aa4721 100644
--- a/Makefile
+++ b/Makefile
@@ -1278,7 +1278,7 @@
 # Directories & files removed with 'make mrproper'
 MRPROPER_DIRS  += include/config usr/include include/generated          \
 		  arch/*/include/generated .tmp_objdiff
-MRPROPER_FILES += .config .config.old .version .old_version \
+MRPROPER_FILES += .config .config.old .version \
 		  Module.symvers tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS \
 		  signing_key.pem signing_key.priv signing_key.x509	\
 		  x509.genkey extra_certificates signing_key.x509.keyid	\